The key finding of the present study is that YM ingestion augments FAO and reduces CHO reliance during exercise, over a wide range of exercise intensities, particularly in the light and moderate domains that are known to be effective training intensities and are often prescribed for a variety of population groups with the aim of weight loss, disease prevention and improving endurance exercise performance[13–15, 18]. Augmented metabolic benefits of YM ingestion when combined with exercise could consequently contribute to the prevention and treatment of overweight and obesity associated metabolic health risks including diabetes and hyperinsulinaemia, hypercholestrolaemia and cancer[19, 20].
In both treatment conditions FAO was increased similarly as a function of power output, but higher FAO was found in the YM condition at exercise intensities below 70% of (Figures 1 and2). Within this range of exercise intensities, FAO is well known to be utilised as a primary fuel source for energy expenditure (30-70%), while CHO predominates at heavy exercise intensities until reaching CHO saturation level (corresponding to RER = 1)[9, 10, 12]. Therefore, increased FAO and EEFAO with YM ingestion in the light and moderate exercise intensity domains may augment both exercise dependent outcomes associated with those intensities[11, 14], and may augment metabolic and anti-adiposity markers associated with YM ingestion, such as decreased differentiation of pre-adipocytes and reduced accumulation of lipids in adipocytes[3–5].
It is well documented that FAO and lipolysis increase several fold during exercise than at rest[10, 21, 22], and the present study is the first to demonstrate the positive effects of YM ingestion during exercise, particularly submaximal exercise, rather than at rest. Only one study reported a reduction in RER during 1–4 hrs of rest in healthy individuals who ingested 1 g of YM, which is similar to the present study. In comparison, the present study, using a similar dosage and following one hr rest, found that the differences in RER in YM vs. PLC become more prominent and significant during exercise (Figure 1), which corresponded to 24% increase in both FAO and EE derived from FAO (Table 1). This increase proposes YM as a promoter of fat metabolism during exercise.
The exercise induced metabolic effects found in the present study could be attributed to a number of reasons related to the major constituents of YM. Perhaps, the main effects YM function found on FAO are explained by central mechanisms and glycogen sparing of caffeine, which is found to have the highest concentration of 1% to 2% (naturally occurring ~1.5% in the present study) of dry weight of YM leaves and stem. This caffeine amount (approximately 80 mg) is considered low compared with other multi-ingredient thermogenic supplements that reported increased FAO and EE (only at rest) where caffeine content exceeds 350 mg, which suggests that other active ingredients may have contributed to the present findings. The remaining reported compounds of YM include saponins, which are attributed to anti-lipolytic and hypocholestrolaemic properties when administered chronically, and caffeoyl derivatives (caffeic acid, chlorogenic acid, 3, 4-Dicaffeoylquinic acid, 4, 5-Dicaffeoylquinic acid and 3, 5-Dicaffeoylquinic acid) which are thought to have higher concentration compared with green or black tea and to have mainly anti-oxidant properties. It has recently been shown that plasma lipid profile (triglycerides, fatty acids and total cholesterol) is ameliorated in mice fed with YM, combined with a positive effect on leptin’s central and peripheral induced feedback loop that regulates adipose tissues, energy balance and EE. However, it is difficult to link these mechanisms with the acute effects during exercise in humans, though recent studies have suggested an acute effect of exercise on changes in lipid profile, which could be augmented by the ingestion of YM. It is also important to note that YM has a number of amino acids (i.e. Glutamic acid, Proline), minerals (P, Fe and Ca) and vitamins (C, B1 and B2) which have energy metabolism properties[1, 3], which could influence exercise metabolic outcomes, though further research is required to determine their potential relationship.
Perhaps the closest supplement to compare the found increase in FAO contribution to total EE is green tea (Camella Sinesis). This is due to several similar active ingredients especially the phenolic antioxidant content in both YM and green tea, though catechin polyphenols have been shown to be highly abundant in green tea, while saponins and some caffeoyl derivatives are abundant in YM[1, 3]. The present study administered 1 g of YM, almost half of the dosage of 1.8 g of green tea extract (no caffeine) administered by Venables et al.[14, 16], and close to the dose of 525 mg of green tea extracts (375 mg catechins and 150 mg caffeine) with similar percentage of 1% of caffeine administered within 24 hrs by Dulloo et al.[19, 29]. In comparison to the effects on EE reported in these studies, and only considering the similar active ingredients to those found in YM, the relatively small dosage of YM administered in the present study seem to be effective in further stimulating FAO and EE contribution to total FAO during exercise intensities in the moderate domain (Table 1), which is in agreement with the latter studies. However, the increase in FAO found in the present study of 0.09 g.min-1 is higher than a ~0.06 g.min-1 increase with green tea ingestion and is close to 0.11 g.min-1 that was reported with an exercise training intervention. Comparative analysis of YM and green tea that have used several measurement techniques have demonstrated higher content of active ingredients (approximately 50 more active ingredients) than those found in green tea[1, 3], which suggest a potential important role for YM in metabolic health, and future research may elicit the potential health outcomes of combining YM with exercise training.
The study relied on an effective exercise protocol that is known reflect a wide range of exercise intensities, including effective training exercise intensities in the light and moderate domains (corresponding to RER ≤ 1) which are considered effective for a number of metabolic health, weight loss and cardiovascular risk reduction outcomes in a variety of population and age groups[14, 30–32]. For example, training at intensities that correspond to maximal FAO has been associated with improved insulin sensitivity and fat loss. Increased reliance on EE from FAO energy fuel sources with approximately 0.5-1.0 kcal.min-1 increase (P < 0.01) in EE from FAO and reduction in EE from CHO as induced by YM compared with PLC (Table 1) reflects a rightward shift in the intensity at the cross-over point defined as the power output when EE from CHO fuel sources predominates over EE from FAO, and hence it reflects an improvement in muscle glycogen sparing capacity, which is a known determining factor for endurance exercise performance. Even though assessing YM effectiveness when combined with exercise training requires further investigations, the present study suggest a potential role for YM in increasing the training effectiveness at the cross-over point intensities[33, 34], that have been tested within this study.
The YM dependent reduction (though non-significant) in BLC during exercise (Figure 4), is indicative of an effect on exercise tolerance and delaying fatigue mechanisms, are also in line with lower reliance on CHO as an energy fuel and increased reliance on FAO at exercise intensities corresponding to RER < 1 (Figure 3), which are all in the submaximal intensity domain, and agree with the dynamic interrelationship between BLC, CHO and FAO[11, 12].
The present study relied on well-established metabolic markers that are based on capillary blood and cardiorespiratory gas measurements to demonstrate the YM effects during carefully selected exercise protocol with several intensity domains[11, 15, 18, 34]. However, future studies need to further investigate the blood-based fat metabolic variables such as glycerol and non-esterified fatty acids, which would confirm the mechanisms behind any putative increase in lipid mobilisation and utilisation. Moreover, assessing both YM and plasma lipids post-ingestion for the active ingredients (eg. caffeic acid, chlorogenic acid, 3, 4-Dicaffeoylquinic acid, 4, 5-Dicaffeoylquinic acid and 3,5-Dicaffeoylquinic acid, phytosterols and saponins) would confirm the bioavailability of YM’s active ingredients.
It is also important to note that the YM effects on FAO and EE within the present protocol, particularly at sub-maximal intensities, may need to be further investigated using a variety of exercise protocols before any exercise training recommendations can be drawn. In particular, combining YM supplementation with supra-maximal and sprint type protocols, and training studies that utilises the recently publicised high-intensity interval training could be further investigated.
To conclude, acute ingestion of YM before exercise enhances fat metabolism during light and moderate exercise intensities, without negatively affecting maximal performance. These effects also suggest a glycogen sparing potential for exercise performance. Further research is required on specific long-term strategies that combine YM with exercise to accelerate weight loss outcomes and potentially enhance metabolic health outcomes.